Saturn and Earth are both planets. Saturn is round, like Earth, but Saturn has fantastic rings, which Earth does not. A planet needs to be round but it doesn't need rings. Both of these bodies orbit a star, another thing planets need to do. That star is our Sun.

Earth Orbits a Star

Certainly no one today doubts that Earth orbits a star, the Sun. Photos taken from space, observations made by astronauts, and the fact that there has been so much successful space exploration that depends on understanding the structure of the solar system all confirm it. But in the early 17th century saying that Earth orbited the Sun rather than the reverse could get you tried for heresy, as it did Galileo. Let’s explore the evolution of the idea that Earth orbits the Sun.

To an observer, Earth appears to be the center of the universe. That is what the ancient Greeks believed. This view is called the geocentric model, or "Earth-centered" model, of the universe. In the geocentric model, the sky, or heavens, are a set of spheres layered on top of one another. Each object in the sky is attached to a sphere and moves around Earth as that sphere rotates. From Earth outward, these spheres contain the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn. An outer sphere holds all the stars. Since the planets appear to move much faster than the stars, the Greeks placed them closer to Earth. The geocentric model explained why all the stars appear to rotate around Earth once per day. The model also explained why the planets move differently from the stars and from each other.

Around 150 A.D. the astronomer Ptolemy resolved this problem by using a system of circles to describe the motion of planets (Figurebelow). In Ptolemy’s system, a planet moves in a small circle, called an epicycle. This circle moves around Earth in a larger circle, called a deferent. Ptolemy’s version of the geocentric model worked so well that it remained the accepted model of the universe for more than a thousand years.

According to Ptolemy, a planet moves on a small circle (epicycle) that in turn moves on a larger circle (deferent) around Earth.

The Heliocentric Universe

Ptolemy’s geocentric model worked, but it was complicated and occasionally made errors in predicting the movement of planets. At the beginning of the 16th century A.D., Nicolaus Copernicus proposed that Earth and all the other planets orbit the Sun. With the Sun at the center, this model is called the heliocentric model, or "sun-centered" model.

Although Copernicus’ model was simpler – it didn’t need epicycles and deferents - it still did not perfectly describe the motion of the planets. Johannes Kepler solved the problem a short time later when he determined that the planets moved around the Sun in ellipses (ovals), not circles (Figurebelow). Kepler’s model matched observations perfectly.

The heliocentric model did not catch on right away. When Galileo Galilei first turned a telescope to the heavens in 1610, he made several striking discoveries. Galileo discovered that the planet Jupiter has moons orbiting around it. This provided the first evidence that objects could orbit something besides Earth.

Galileo also discovered that Venus has phases like the Moon (Figurebelow), which provides direct evidence that Venus orbits the Sun.

The phases of Venus.

Galileo’s discoveries caused many more people to accept the heliocentric model of the universe, although Galileo himself was found guilty of heresy. The shift from an Earth-centered view to a Sun-centered view of the universe is referred to as the Copernican Revolution.

In their elliptical orbits, each planet is sometimes farther away from the Sun than at other times. This movement is called revolution. At the same time, Earth spins on its axis. Earth’s axis is an imaginary line passing through the planet’s center that goes through both the North Pole and the South Pole. This spinning movement is called Earth’s rotation.

Earth’s Revolution

Copernicus, Galileo, and Kepler were all right: Earth and the other planets travel in an elliptical orbit around the Sun. The gravitational pull of the Sun keeps the planets in orbit. This ellipse is barely elliptical; it's very close to being a circle. The closest Earth gets to the Sun each year is at perihelion (147 million km) on about January 3rd, and the furthest is at aphelion (152 million km) on July 4th. The shape of Earth’s orbit has nothing to do with Earth's seasons.

Earth and the other planets in the solar system make elliptical orbits around the Sun.

For Earth to make one complete revolution around the Sun takes 365.24 days. This amount of time is the definition of one year. Earth has one large moon, which orbits Earth once every 29.5 days, a period known as a month.

Summary

In the geocentric model of the universe, Earth is at the center.

In the heliocentric model of the universe, the Sun is at the center. The heliocentric model is the modern view of the solar system, but not the entire universe.